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→Calculate drawdown time
*Begin the drainage time calculation by dividing the area of the practice (''A<sub>p</sub>'') by the perimeter (''P'').
*Begin the drainage time calculation by dividing the area of the practice (''A<sub>p</sub>'') by the perimeter (''P'').
*Use the following equation to estimate the time (''t'') to fully drain the facility:
*Use the following equation to estimate the time (''t'') to fully drain the facility:
:<math>t=\frac{nA_{p}}{f'P}ln\left [ \frac{\left (d_{T}+ \frac{A_{p}}{P} \right )}{\left(\frac{A_{p}}{P}\right)}\right]</math>
:<math>t=\frac{nA_{p}}{f'x}ln\left [ \frac{\left (d_{T}+ \frac{A_{p}}{x} \right )}{\left(\frac{A_{p}}{x}\right)}\right]</math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*''n'' is the porosity of the fill materials within the practice, depth weighted mean
*''n'' is the porosity of the fill materials within the practice, depth weighted mean
*''A<sub>p</sub>'' is the area of the practice (m<sup>2</sup>),
*''A<sub>p</sub>'' is the area of the practice (m<sup>2</sup>),
*''f''' is the design infiltration rate (mm/hr),
*''f''' is the design infiltration rate (mm/hr),
*''P'' is the perimeter of the practice (m), and
*''x'' is the perimeter of the practice (m), and
*''d<sub>T</sub>'' is the total depth of the practice, including the ponding zone (m).}}
*''d<sub>T</sub>'' is the total depth of the practice, including the ponding zone (m).}}